The invasion last March also damaged the credibility of the countries which participated and the authority of the UN Security Council, Blix said in a speech at the University of Edinburgh on Tuesday.
"The justification for the war - the existence of weapons of mass destruction - was without foundation," Blix told an audience of 1000.
In taking armed action, the US and Britain "ignored the views of the majority" on the Security Council, leading to a "loss of legitimacy" for the invasion, he said.
"The states which we would have expected to support and strengthen some basic principles of the UN order, in my view, set a precedent of ignoring or undermining this order by acting too impatiently and without the support of the Security Council.
"The states which we would have expected to support some basic principles of the UN order, in my view, set a precedent of ignoring or undermining this order by acting too impatiently and without the support of the Security Council"
Ex-UN chief weapons inspector
"As a result, their own credibility has suffered and the authority of the Security Council has been damaged," said Blix, speaking as part of a public lecture on the means of reducing the spread of weapons of mass destruction.
Blix added that deposed Iraqi president Saddam Hussein "was dangerous to his own people but not a great, and certainly not an immediate, danger to his neighbours and the world".
Under pressure to explain the failure to uncover convincing proof of banned weapons in Iraq, both London and Washington have ordered inquiries into apparent failures by their intelligence services.
Blix, a former Swedish foreign minister who was charged with searching for weapons of mass destruction in the 15 weeks leading up to the US-led invasion, never hid his scepticism that Iraq actually had such weapons.
US lawmakers pressed CIA Director George Tenet on Tuesday on recent comments he made about having told the White House that prior to the war Iraq had not posed an imminent threat.
George Tenet faced a Senate
panel over his comments on Iraq
"If it wasn't an imminent threat in your mind, how would you have characterised or assessed the threat at that point in time?" asked Republican Senator Olympia Snowe.
"I would have characterised it as something that was grave and gathering, something that we were quite worried about," Tenet told the hearing of the Senate Intelligence Committee on the threat to national security.
Snowe noted President George Bush's words in his 17 September 2002 speech on his national security strategy.
"When the threat is imminent, the nation has the right to conduct preemptive operations."
Refusing to state that Bush launched the Iraq war due to a threat that was less than imminent, Tenet said the CIA "said things quite assertively in our key judgments that caused the policy-maker to have a look at this thing in a way that he or she had to assess risk".
"Those are just the facts as we know them today," Tenet said.
"We can go back and, of course, we will look at all of this work. And make judgments about did we word everything carefully, did we have the right context and everything.
"That's appropriate. We need to do that as professionals."